The present invention relates to a cellular telephone apparatus, particularly to a portable subscriber set suitable for use in the Advanced Mobile Phone Service (AMPS) and/or the Total Access Communication System (TACS) which is based on AMPS.
AMPS is disclosed in detail in the Bell System Technical Journal, January 1979 Vol 58, No. 1 and in the interest of brevity only those aspects pertinent to the present invention will be discussed. In order to facilitate an understanding of the present invention a brief description will be given of some of the data communications in the "forward" direction, that is from a base station at a cell site to a subscriber set, and in the "reverse" direction, that is from a subscriber set to a base station.
Each cell in a mobile service area is assigned a batch of duplex channels which is different from the batches allocated to neighbouring cells. The two channels of each duplex pair are separated from each other by 45 MHz. However as the overall number of channels is limited, batches of duplex channels are reused in cells geographically separated from each other so that the risk of signals transmitted by one cell site being mistaken by subscriber sets operating in another distant cell which is allocated the same batch of channels is slight. In order to reduce this risk further a system of supervisory audio tones (SATs) is used which are transmitted by base stations and transponded by the subscriber sets.
Within each cell, one channel of the batch of channels is reserved for setting-up voice channels between subscriber sets and the cell site in which they are currently present. This channel is termed the set-up or access channel and forward and reverse signalling on this channel is exclusively data. Once a voice channel has been set-up data is also transmitted by a "blank and burst" technique in which the voice signal is blanked and data is sent rapidly in a burst. Details of all the possible message formats on the control and voice channels can be derived from for example Bell Systems Technical Journal, January 1979 and the Cellular Mobile Telephone Equipment Specification, July 1982 issued by Advanced Mobile Phone Service Inc. However, four of the more pertinant message formats will be described with reference to FIGS. 1 to 4 of the accompanying drawings.
FIG. 1 shows the forward control channel (FOCC) message stream which consists of three discrete information streams called stream A, stream B and busy/idle stream denoted by the arrows. Each mobile apparatus monitors stream A or stream B and the busy/idle stream.
The message begins with a 10-bit dotting sequence (1010101010) D for bit synchronization, and is followed by an 11-bit word synchronization sequence (11100010010) WS to achieve synchronization with the incoming data. Each word is encoded and contains 40 bits, including parity and is repeated five times. The coding is a linear systematic block code, particularly a (48, 28) BCH code.
For ease of reference the five transmissions of the words A and B have be referenced A1 to A5 and B1 to B5.
FIG. 2 shows the forward voice channel (FVC) data stream, its data rate being 10 kb/sec for AMPS and 8 kb/sec for TACS. The datastream commences with a 101-bit dotting sequence D(101) followed by an 11-bit word synchronization sequence and the first repeat of the word W1. Each word encoded into a (40,28) BCH linear systematic block code is repeated ten more times, referenced W2 to W11; each repeat being preceded by a 37-bit dotting sequence and the 11-bit word synchronizing sequence.
FIG. 3 illustrates the data stream on the reverse control channel (RECC). The data stream is discontinuous and the data rate is 10 kb/sec for AMPS and 8 kb/sec for TACS. The RECC data stream comprises 30 bits of dotting sequence, D, an 11-bit word synchronization sequence, WS, and a 7-bit coded digital color code CDCC (see the Cellular Mobile Telephone equipment Specification referred to above for further information) followed by a 48 bit first word repeated five times (5.times.RW1), a 48 bit second word repeated five times (5.times.RW2), a 48 bit third word repeated five times (5.times.RW3) and so on. The dotting sequence, D, the word synchronization sequence WS and the CDCC constitute a seizure precursor SP. The 48 bit words are encoded in accordance with a BCH (48,36) linear systematic block code.
Finally FIG. 4 shows the reverse voice channel (RVC) data stream which from time to time interrupts analog voice signals. The data stream commences with a 101 bit dotting sequence D(101) followed by an 11-bit word synchronization sequence and the first repeat of a first 48 bit word 1 RW1. Thereafter there are four more repeats of the first word, denoted by 2RW1 to 5RW1, each repeat being preceded by a 37 bit dotting sequence and an 11 bit word synchronization sequence. This data stream is followed by 37 bits of dotting sequence and 11 bits of word synchronisation sequence with five repeats of a second word 1RW2 to 5RW2, and so on.
In a subscriber set, which term includes portable and mobile subscriber sets, it has been customary to digitally process all the data and control information received or to be transmitted as well as provide the internal control information for the subscriber set by means of a microprocessor logic unit. Many program steps are needed in order to be able to implement the required functions in software which in turn means a high clock frequency and a high current consumption. However other sections of the subscriber sets operate at other clock frequencies and in consequence it is necessary to provide several clock generators formed by separate oscillators in each set. A drawback to having several oscillators is that they are expensive, current hungry and are sources of spurious signals.
An object of the present invention is to provide an architecture for a cellular telephone apparatus which mitigates these drawbacks.